A. Technical Field
The present invention relates to a water-absorbing agent and its production process and use, more particularly, relates to a water-absorbing agent of excellent urine resistance, especially, a water-absorbing agent which can always exhibit excellent absorption properties regardless of the kinds of liquids, such as urine, to be absorbed, and a production process for the water-absorbing agent, and further relates to uses of the water-absorbing agent, namely, to absorbent matters and articles, and still further relates to an absorption property measurement process by which absorption actions can easily and precisely be predicted when the water-absorbing agent and the absorbent matters and articles are practically used.
B. Background Art
In recent years, water-absorbent resins (water-absorbing agents) are widely used as constituent materials of sanitary materials, such as paper diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids such as urine and menstrual blood.
Known examples of the above water-absorbent resins are as follows: crosslinked polymers of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile copolymers or acrylamide copolymers, and their crosslinked polymers; and crosslinked polymers of cationic monomers.
It is said that the above water-absorbent resins should, for example, have the following properties: excellent water absorption amount and speed, the gel strength, the suction power to suck up water from a base material containing an aqueous liquid, upon contact with aqueous liquids such as body fluids. However, there are problems in that relations between these properties do not necessarily display positive correlations: for example, as the absorption capacity increases, some other properties such as liquid-permeability, gel strength, and absorption speed deteriorate.
As to a method for improving such water-absorption properties of the water-absorbent resin in good balance, an art is known, in which the neighborhood of the surface of the water-absorbent resin is crosslinked, and various methods have been proposed as such.
For example, methods are known, in each of which the following materials are used as the crosslinking agents: polyhydric alcohols (JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-59-189103); polyvalent metals (JP-A-51-136588, JP-A-61-257235 and JP-A-62-007745); monoepoxy compounds (JP-A-61-098121); epoxy compounds and hydroxy compounds as used jointly (JP-A-02-132103); alkylene carbonates (DE 4020780).
However, there are problems in that: the balance between the water-absorption properties is being improved by the above surface treatments, but when the water-absorbent resin is used for absorbent matters of diapers, the water-absorbent resin deteriorates with time, and the liquid-permeability or the gel strength falls, so urine leaks from the diapers. The deterioration of the water-absorbent resin occurs from the surface of the water-absorbent resin, and the soluble contents elute, and the liquid-permeability or the gel strength falls. Such a deterioration of the water-absorbent resin is considered to be caused by a very small amount of metal ion and by L-ascorbic acid as contained in urine.
By the way, the water-absorbent resin is powdery and therefore might contain fine powders of 100 μm or less, and it is known to make a granulation by adding water for the purpose of improving the handling ability or the liquid-permeability in diapers. The granulation can prevent the powdering or improve the fluidity during moisture absorption.
However, there are problems in that the granulation by adding water to the surface-crosslinked water-absorbent resin facilitates the destruction of the surface-crosslinked layer. Especially, as to water-absorbent resins with high absorption capacity under load as desired in recent years, the elution of the soluble contents is prevented by crosslinking the neighborhood of the surface of water-absorbent resins with high absorption capacity, so the elution of the soluble contents cannot be suppressed in the case where the surface-crosslinked layer is deteriorated by substances such as L-ascorbic acid when absorbing urine. Therefore, there are problems in that when the water-absorbent resin is used for diapers, the liquid-permeability or the gel strength deteriorates, so urine leaks from the diapers.
On the other hand, as to uses of the water-absorbent resin, a variety of absorbent matters or articles using water-absorbent resins are proposed, wherein the water-absorbent resins jointly have a plurality of the aforementioned properties and exhibit excellent performance (water absorption properties) when used for sanitary materials such as paper diapers and sanitary napkins.
For example, the following are known: a water-absorbent resin comprising combinations of a gel capacity, a shear elastic modulus, and an extractive polymer content as are specified (U.S. Pat. No. 4,654,039); a water-absorbent resin with a water absorption amount or speed and a gel strength as are specified, and paper diapers and sanitary napkins using this water-absorbent resin (JP-A-60-185550, JP-A-60-185551, and JP-A-60-185804); paper diapers using a water-absorbent resin having a specific water absorption amount or speed and a gel stability (JP-A-60-185805); water-absorbent articles using a water-absorbent resin with a water absorption amount, a suction power, and a water-soluble content as are specified (JP-A-63-021902); water-absorbent sanitary supplies containing a water-absorbent resin with a water absorption amount, a water absorption amount under a load, and a gel fracture strength as are specified (JP-A-63-099861); paper diapers containing a water-absorbent resin with a water absorption amount and a water absorption speed under a load as are specified (JP-A-02-034167); a water-absorbing agent containing a water-absorbent resin with a water absorption amount under a load and a particle diameter as are specified (EP 339,461); a water-absorbing agent containing a specific or larger amount of water-absorbent resin with a water absorption speed and a water absorption amount under a load in a short time as are specified (EP 443,627); a water-absorbent combined material containing a specific or larger amount of water-absorbent resin with a deformation under a load and a suction index as are specified (EP 532,002); and an absorbent article using a resin with a pressure absorption index and a 16-hour extractability level as are regulated (EP 615,735).
In recent years, absorbent articles such as paper diapers are getting thinner and thinner, and the amount of the water-absorbent resin, as used for an absorbent layer of the absorbent articles, tends to increase. That is to say, as to the above absorbent layer, what has the weight ratio of 0.3 or more, particularly, 0.5 or more, of the water-absorbent resin to the total of the water-absorbent resin and the fibrous base material (this ratio might hereinafter be referred to as “resin concentration”) is becoming the mainly current. However, it is becoming clear that there are, still, problems when the above already known resins with a variety of regulated properties are used for these absorbent articles having high resin concentration. That is to say, the water absorption properties of the absorbent articles are being improved by combinations of the above various properties, but it is being closed up that there are problems in that, depending on the composition of the liquid to be absorbed, the water absorption properties of the resins cannot be sufficiently displayed especially when the resin concentration in the absorbent articles is high. It is being said that there are problems in that, when the absorbent article is, for example, a paper diaper, the composition of urine varies with factors, such as users' ages, taken food and drink, prescribed medicines, and in that the absorption action of the water-absorbent resin might therefore be greatly different from expectation.